Control systems often employ servo controlled hydraulic valves to control hydraulic line pressure. For example, one application of a servo controlled hydraulic valve is for controlling differentially driven wheel slip in vehicles. Wheel slip is controlled by selectively activating the vehicle's hydraulically operated brakes in a manner which directs braking power to the slipping wheel or wheels. An electrical control system senses wheel slippage and produces pulse width modulated control signals whose duty factors are proportional to the desired response of a hydraulic proportioning valve. The proportioning valve is in turn connected to a control valve which controls the pressure of hydraulic fluid applied to the brakes.
Servo controlled valves of the type mentioned above include an electrically responsive coil for controlling an output of the valve. These coils are selectively energized with a suitable voltage source for time periods corresponding to the periods of the control signals. As will be discussed more fully hereinafter, substantial variations in valve response are experienced as a result of changes in both power supply voltage and the resistance of the coil due to temperature variations. In some cases, valve response, which may be measured in terms of the magnitude of hydraulic output pressure from the valve, may vary considerably. This pressure variation significantly reduces the accuracy of a control system employing servo valves and may prevent their use in some applications.
The present invention is directed to overcoming the problems associated with the prior art discussed above and provides a significantly improved pulse width modulated servo driver which affords consistently repeatable valve response.